Method for gas absorption across a membrane

ABSTRACT

The invention relates to a method for the absorption of one or more gaseous components from a gas phase, in that the gas phase with the component(s) to be absorbed present therein is brought into contact with a liquid phase, wherein the gas phase and the liquid phase are separated by a hydrophobic membrane of a material other than polytetrafluoroethene, wherein the liquid phase comprises water and a water-miscible and/or water-soluble absorbent, and wherein the liquid phase does not give rise to any leakage from the membrane or is effective in preventing or counteracting leakage from the membrane. According to a first preferred aspect, the liquid phase comprises waterand a water-miscible and/or water-soluble organic absorbent, wherein the surface tension at 20° C. has been brought to at least 60×10 -3  N/m by adding a water-soluble salt. According to a second preferred aspect, the liquid phase comprises an aqueous solution of a water-soluble amino acid or a salt thereof, such as taurine and derivatives. According to a third preferred aspect, the liquid phase comprises an aqueoussolution of a water-soluble phosphate salt. The membranes are preferably in the form of hollow fibres of, for example, polypropene or polyethene. The method is suitable in particular for the absorption of carbon dioxide.

FIELD OF THE INVENTION

The invention relates to a method for the absorption of one or moregaseous components from a gas phase, in that the gas phase with thecomponent(s) to be absorbed present therein is brought into contact witha liquid phase, wherein the gas phase and the liquid phase are separatedby a hydrophobic membrane of a material other thanpolytetrafluoroethene, and wherein the liquid phase comprises water anda water-miscible and/or water-soluble absorbent.

BACKGROUND OF THE INVENTION

A method of this type was disclosed in the now withdrawn European patentapplication 0 451 715 (H. Matsumoto et al.) in the name of Mitsubishi,claiming a Japanese priority from 1990. The reference describes anapparatus for separating polar gases from a gas source, in which saidgas source containing the polar gas(es) is led through an encased moduleof porous hollow membrane filaments made of hydrophobic material andhaving innumerable micropores penetrating through the wall of the hollowfilament and distributed over the wall of the hollow filament foreffecting gas/liquid mass transfer to an absorption liquid flowing onthe other side of the hollow filaments of the module. Although thisreference is preferably directed to an apparatus for carrying out saidmethod, it is also described that as the absorption liquid"diethanolamine, aqueous solutions of K₂ CO₃ and KHCO₃.H₂ O and mixturesthereof" can be used. However, this reference is silent with respect tothe parameters for carrying out this process, including theconcentrations of the components in the absorption liquor, and thesurface tension of the final solution.

In a 1994 article from the inventors of EP-A-0 415 715 entitled"Fundamental Study on CO₂ Removal from the Flue Gas of Thermal PowerPlant by Hollow-Fibre Gas Liquid Contactor", Mitsubishi Heavy IndustriesLtd and Tokyo Electric Power Company, presented at CO₂ ChemistryWorkshop, Hemavan, Sweden, 19-23 Sep. 1993, results are given for theabsorption from carbon dioxide from the flue gas from power stationswith the aid of hollow fibre membranes, the liquid phase used being asolution of monoethanol amine in water.

In this article, a comparison is given of polypropene (PP), polyethene(PE) and polytetrafluoroethene (PTFE) hollow fibres, from which it canbe seen that with polypropene and polyethene membranes the mass transfercoefficient decreases after some time in the case of continuous use.Consequently, using these membranes it is not possible to operategas/liquid absorption under stable conditions over a prolonged period.Therefore, in this article preference is given to PTFE hollow fibremembranes.

However, the PTFE membranes used have an appreciably lower mass transfercoefficient than the polypropene and polyethene membranes. Moreover,PTFE is very difficult to process. For example, hollow fibres with smallexternal diameters (<0.72 mm) which are desirable for use in compactequipment for, for example, the `offshore` industry and aerospace,cannot be produced from PTFE.

As it can be seen from the above two citations, the inventors/authorsthereof, despite the four years lying between these references, wereunable to satisfactorily perform gas/liquid absorption with polypropeneor polyethene hollow fibres, concentrating instead on PTFE fibres, whichhowever have practical disadvantages.

The first aim of the invention is, therefore, to improve theabovementioned gas/liquid absorption methodology, more particularly toprovide a method for membrane gas absorption which can be operated understable conditions over a prolonged period in a system of smalldimensions.

A compact system of this type could be provided by the use of, forexample, polypropene or polyethene hollow fibres in place of PTFE hollowfibres, in combination with an aqueous solution of a conventionalabsorbent, such as monoethanolamine.

However, research by the Applicant has now revealed that when an aqueoussolution of an organic absorbent customary in the prior art, (such asmonoethanolamine/water mixtures) is used with polyethene or polypropenehollow fibre membranes, not only do the abovementioned problems withregard to the reduction in the mass transfer coefficient arise butleakage also occurs, a problem which is not recognised in theabove-mentioned prior art.

A second aim of the invention is, therefore, to prevent and/orcounteract leakage in membrane gas absorption, in particular with hollowfibre membranes, as a result of which it becomes possible to operatemembrane gas absorption under stable conditions for a prolonged periodusing, for example, polypropene or polyethene hollow fibre membranes.

A third aim of the invention is to provide liquid phases for use inmembrane gas absorption, which produce no leakage and/or are effectivein preventing and/or counteracting leakage, and which, at the same time,give acceptable or improved characteristics relevant for membrane gasabsorption, such as mass transfer, kinetics, regeneration energy and/orcorrosiveness, individually and in combination.

A fourth aim of the invention is to provide a system for carrying outmembrane gas absorption which has small dimensions, is reliable and doesnot display any leakage, and nevertheless gives good and rapid removalof the gaseous components to be removed, with which method polypropeneand/or polyethene hollow fibres of small dimensions are preferably used.

Further aims of the invention will become apparent from the followingdescription.

The international patent application 9401204 describes a membraneseparation process for dehydrating a gas or vapour or liquid mixture bypervaporation, vapour permeation or gas separation, i.e. by using amembrane, for instance in the form of hollow fibres. As the membranematerial, polyphenylene oxide and a flat polyvinyl alcohol composite areillustrated, without for instance polyethene or polypropene explicitlybeing mentioned.

According to this reference the absorption liquid is a highlyconcentrated solution of one or more hygroscopic salts, with ahygroscopic capacity higher than 50%, preferably higher than 80%, suchas brines based on LiBr, CSF, KC₂ H₃ O₂, MgCl₂ and mixtures thereof, forexample LiBr/ZnBr₂ /CaBr₂. (It is mentioned that sodium chloride orpotassium dichromate will not exhibit the desired effect, and carbonatesare not mentioned at all.)

Furthermore, although it is mentioned that it was not possible to detectany leakage of brine with a LiBr/ZnBr₂ /CHBr₂ brine and a flat polyvinylalcohol composite membrane, this reference does not acknowledge theproblems of leakage of membranes in general, such as polyethene orpolypropene membranes, when used in combinations with aqueous solutionsof monoethanol amine as the liquid phase. Also, it gives no indicationthat such problems could be overcome by adding one or more water-solublesalts in concentrations that are much lower than in the highlyconcentrated hygroscopic solutions according to this reference.

Finally, this reference only relates to the absorption of water vapour;the absorption of for instance carbon dioxide or hydrogen sulfide isneither mentioned nor suggested.

The international application 9401204 describes a method and device forregulating the humidity of a gas flow and at the same time purifying itof undesired acid or alkaline gases. According to this method a membranemodule is used that contains one or more membranes which are microporousand hydrophobic, for instance comprising hollow fibres made ofpolypropene. As the absorption liquid, again a hygroscopic liquid isused, consisting of polar glycols, alcohols or glycerols such astriethene glycol or polyethene glycol or mixtures thereof, or ahygroscopic liquid consisting of a watery electrolyte solution withhygroscopic qualities, for instance mixtures of these glycols and sodiumor potassium carbonate solutions, i.e. in a ratio of 4:1 on a weightbasis. These absorption liquids, despite containing a small amount ofaqueous carbonate solutions, are therefore of an organic nature, theconcentration of the organic absorption component being well over 10M.

However, the use of aqueous solutions of organic amines andwater-soluble salts is neither mentioned nor suggested, nor the usethereof in preventing leakage with for instance polypropene orpolyethene hollow fibres.

The U.S. Pat. Nos. 5,281,254 and 4,954,145 describe methods for gasabsorption using porous membranes, the pores of which are filled with anabsorption liquid. These references are related to gas/gas absorptioninstead of gas/liquid absorption. Also the pores of the membranes arenot filled with aqueous solutions, but with, for instance, organicamines.

U.S. Pat. No. 4,147,754 describes the use of "immobilized liquidmembranes" in the gas/gas absorption of H₂ S.

Therefore, none of the abovementioned references is concerned with theproblem of leakage of aqueous absorption liquids when used incombination with, for instance, polypropene or polyethene hollow fibres,and no solution for this problem is mentioned or suggested. The presentinvention for the first time acknowledges this problem, and offers asolution in that the liquid phase is so chosen that the membrane gasabsorption can be operated under stable conditions for a prolongedperiod without leakage occurring.

SUMMARY OF THE INVENTION

To this end the invention provides various liquid phases which absorbthe gaseous components to be removed from the gas phase and whichproduce no leakage from the membrane or are effective in preventingleakage from the membrane. Some of these liquid phases are known per seas liquid phases for gas/liquid absorption, for example in columnapparatus.

However, the liquid phases used according to the invention, have not yetbeen used in membrane gas absorption for the prevention of leakage, aproblem which, as has already been mentioned, is also not recognised assuch in the prior art.

In the broadest sense, the invention therefore relates to a method ofthe type described in the preamble, characterised in that a liquid phaseis used which does not give rise to any leakage or is effective inpreventing or counteracting leakage.

A first preferred aspect of the invention is characterised in that theliquid phase comprises water and a water-soluble or water-miscibleorganic absorbent, wherein the surface tension of the liquid phase at20° C. has been brought to at least 60×10⁻¹ N/m by the addition of awater-soluable salt.

A second preferred aspect of the invention is characterised in that theliquid phase comprises an aqueous solution of a water-soluble amino acidand/or a water-soluble salt of an amino acid.

A third preferred aspect of the invention is characterised in that theliquid phase comprises an aqueous solution of a water-soluble phosphatesalt.

Further preferred aspects of the invention and the advantages thereofwill become apparent to those skilled in the art from the text whichfollows.

However, it must be understood that the invention is not restricted tothe three preferred aspects but that any liquid phase, preferablyaqueous liquid phase, which absorbs the desired gaseous impurity andwhich is effective with regard to the prevention and/or counteraction ofleakage in membrane gas absorption falls under the scope of theinvention. On the basis of what is described in the present application,those skilled in the art will be able to determine when leakage occursand which liquid phases produce no leakage or are effective inpreventing and/or counteracting this.

From the standpoint of the further characteristics relevant for membranegas absorption, such as mass transfer, kinetics and/or regenerationenergy, corrosiveness and the like, the three abovementioned liquidphases are, however, to be preferred, as will become apparent from thedescription below.

In this context, the invention provides several alternative liquidphases, all of which can be used for membrane gas absorption withoutleakage being obtained. As a result, a person skilled in the art isplaced in the position of being able, by the selection of the liquidphase and the further conditions, to provide a membrane gas absorptionsystem which is as optimum as possible for the desired application,further factors, such as the gaseous impurity to be removed, themembrane used, the equipment used, the desired degree of removal, thetemperature, the desired mass transfer and kinetics, the method ofregeneration and the like, playing a role. In particular and preferably,the liquid phases of the invention will display good kinetics and highmass transfer, which remain constant even in the case of continuousoperation over a prolonged period. In particular it is possible, usingthe liquid phases of the invention, to prevent an undesired reduction inthe mass transfer with time, as described in the article by Matsumoto etal, described above, for the use of monoethanolamine/water solutionswith polypropene or polyethene hollow fibres.

Furthermore, it is also possible for various elements of theabovementioned preferred aspects to be combined, such combinationsfalling within the scope of the invention. For instance it is possible,for example, to use a liquid phase which comprises an aqueous solutionof both a water-soluble phosphate salt and a water-soluble amino acid ora salt thereof, or, for example, a phosphate salt of a water-solubleamino acid. It is also possible to use a phosphate salt, a water-solubleamino acid or water-soluble salt thereof in order to bring the surfacetension, at 20° C., of an aqueous solution of an organic absorbent inwater above the value required according to the invention. Furtherpossible combinations will be apparent to those skilled in the art.

Furthermore, the fact that not every liquid phase is equally suitablefor every conceivable application also falls within the scope of theinvention. A person skilled in the art would, however, be able to selecta suitable combination of liquid phase, membrane material and conditionsfor the desired application from the alternatives offered according tothe present application.

The invention is used, in the broadest sense, with membranes whichdisplay leakage with known liquid phases, such as, for example, aqueoussolutions of conventional organic absorbents.

According to the present Application, leakage must be understood to meanthe undesired permeation of the liquid absorbent through the membrane.As a result, the pores of the membrane become filled and/or moistenedwith the liquid absorbent and in serious cases the essential action ofthe membrane with regard to keeping the gas phase and the liquid phaseseparate can be adversely affected.

However, the invention is not restricted to a specific mechanism or aspecific explanation for the occurrence of the leakage. The Applicanthas found that whether or not leakage occurs is related to variousfactors in the absorption system, including the membrane used and theliquid phase used.

As a result of the occurrence of this leakage it is possible that theabsorbent effect of the membrane system decreases or is even lostcompletely, the separation efficiency is reduced or the equipmentbecomes blocked and/or damaged. It can be seen from this that leakagecan be a serious problem which can adversely affect the operation andreliability of a membrane gas absorption system, which specifically inapplications wherein a high reliability is desired--such as in theoffshore industry and aerospace--is highly undesirable.

With conventional liquid absorbents, the leakage which the inventionaims to counteract or to prevent occurs in general only after the systemhas already been in continuous operation for some time--varying from afew hours to several days. This makes the occurrence of this leakage themore surprising, since it would be expected that leakage would occurimmediately when the membrane is brought into contact with the liquidabsorbent, essentially because the effect of the membrane isinsufficient to keep the liquid phase and the gas phase separate.

In practice, however, it has been found that in the case of continuousoperation of the absorption unit leakage occurs only after some time, sothat it is not possible to predict on the basis of experiments of shortduration whether a specific liquid absorbent is effective in preventingor counteracting said leakage. This again shows the importance of therecognition of the problem of the occurrence of leakage, which has ledto the present invention.

The occurrence of leakage can be established visually or in any othersuitable way, such as will be apparent to those skilled in the art. Onceit has been established that a membrane system is displaying leakage, itis then possible to select and use a suitable liquid phase according tothe invention.

DETAILED DESCRIPTION OF THE INVENTION

The method and liquid phases of the invention will in general be usedwith membranes of a material other than polytetrafluoroethene, such aspolypropene (PP), polyethene (PE), polyvinylidene fluoride (PVDF) andpolysulfone (PSU). Furthermore, the invention can be employed withcoated or treated membrane systems, such as plasma membranes, membranescoated with siloxane rubbers (PDMS), membranes treated with fluorine,paraffins and the like, if leakage also occurs.

The membranes can be used in any desired form, such as in the form offlat membranes with transport channels, the so-called "plate and frame"modules (both in co-current and in counter-current, which is to bepreferred), or in the form of spirally wound flat membranes, as will beapparent to a person skilled in the art.

With membranes in the form of flat fibres of, for example, polypropeneor polyethene, it is possible to achieve the same packing density as isachieved with the polytetrafluoroethene hollow fibres of the prior art.Polypropene and polyether membranes are, however, less expensive andsimpler to produce than polytetrafluoroethene hollow fibres.

The invention is, however, preferably and advantageously used formembrane gas absorption with the aid of hollow fibres, more particularlyhollow fibres of small diameter, which can be processed to produceand/or be used in equipment of small dimensions.

Hollow fibre membranes of this type, the production and the use thereofare generally known in the specialist field. For instance, U.S. Pat. No.4,286,279 describes the use of hollow fibre membranes for gas/liquidabsorption for use in an artificial lung, but without any mention beingmade of problems with regard to the stability or leakage of the membranein the case of continuous use over a prolonged period.

In principle, the use of hollow fibre membranes as contact medium canreduce the dimensions of an absorption device in that large exchangesurface areas (>1000 m² /m³) are achievable with commercially availablemembranes. Compared with conventional packed columns which usually havea specific surface area of about 100 m² /m³, this is appreciablygreater. As a result, significant reductions in the size of theequipment can be achieved.

In addition, there are additional advantages:

completely free choice of the ratio of gas to liquid flow rates;

no entrainment, flooding or foaming;

low pressure drop on the gas side, low percentage flow surface withmembranes;

low liquid hold-up;

counter-current operations readily adjustable with the aid of internallyswitched segments.

With the method according to the invention, the membranes are thereforepreferably hollow fibre membranes produced from an inert porous materialother than PTFE. Said hollow fibres advantageously have an externaldiameter of less than 1 mm, for example 0.33 mm. Such small fibres canat present not be achieved at all with PTFE.

Materials for the production of hollow fibre membranes of this type,such as polypropene (PP), polyethene (PE), polytetrafluoroethene (PTFE),polyvinylidene fluoride (PVDF) and polysulfone (PSU) are known from theprior art. Hollow fibre membranes produced from these materials areoften available commercially as replaceable modules, which, for example,comprise sintered fibres with a porosity of 40-70%.

The invention therefore provides a method for the operation ofgas/liquid absorption with the aid of hollow fibre membranes, which hasa high efficiency, i.e. a high specific surface area and a high masstransfer coefficient and, moreover, in addition no undesired reductionin the mass transfer with time is detected.

The invention also makes it possible to use polypropene and polyethenehollow fibres, which not only have a low cost price but also can havesmaller external diameters than the known PTFE fibres. On the basishereof, the method according to the invention can be operated in compactequipment.

The method of the invention can be used for the removal of multifariousdifferent impurities from gas phases and, in the broadest sense, is notrestricted to specific impurities.

For instance, membrane gas absorption is a very suitable technique forthe removal of, for example, carbon dioxide from gas phases such as air,flue gases and off-gases, which constitutes a preferred aspect of theinvention.

Important aspects when selecting the specific process are, in thiscontext, the CO₂ concentration, the desired degree of removal, thedegradation by oxygen present in the off-gas, the corrosiveness, thedesired purity of the CO₂ produced and the prevailing process conditionsin the off-gas stream.

Especially for applications such as in the offshore industry andaerospace, small dimensions, a low weight and a high reliability (i.e.stability) of the equipment are important. In addition to the availablecontact surface area, in this context the kinetics of the process arealso a decisive factor in determining the volume taken up by theequipment. In this context, the invention makes it possible for a personskilled in the art to obtain the desired characteristics in the ultimatesystem without the occurrence of leakage or an undesired reduction inthe mass transfer.

By correct choice of the liquid phase, the invention can also be usedfor the removal of, for example, hydrogen sulfide or water vapour fromgas phases. The invention can also be used to prevent leakage indesorption processes where membrane techniques are used, in which casethe regeneration energy of the liquid phase can also be an importantprocess parameter.

The invention also relates to a system for membrane gas absorption,comprising a membrane module with hollow fibres of a material other thanpolytetrafluoroethene and a container, which contains an aqueous liquidphase, wherein the liquid phase does not produce any leakage from themembrane or is effective in preventing or counteracting leakage from themembrane. The invention will be discussed in more detail below withreference to the abovementioned preferred aspects and non-limitingexamples.

Finally, it should be understood that the absorption liquids of theclasses described in the present application can of course also be usedwith PTFE membranes, because of their advantageous absorption propertiesdescribed herein.

A. Combination of an organic solvent and a water-soluble salt

According to the first preferred aspect of the invention, a liquid phaseis used which comprises water and a water-soluble or water-miscibleorganic absorbent, wherein the surface tension of the liquid at 20° C.has been brought to at least 60×10⁻¹ N/m by the addition of awater-soluble salt.

In this context the water-soluble salt is preferably chosen fromwater-soluble carbonates, preferably potassium carbonate and sodiumcarbonate. The salt is advantageously used in a concentration of0.05-10M, preferably 0.1-5M.

The organic absorbent is preferably chosen from monoethanolamine (MEA),diethanolamine (DEA), methyldiethanolamine (MDEA), methyl ethyl ketone(MEK), methyl isobutyl ketone (MIBK), (poly)ethene glycols, ethers,alcohols and N-methylpyrrolidone, more preferentially frommonoethanolamine or diethanolamine.

The organic absorbent is advantageously used in a concentration of0.05-10M, preferably 0.1-5M.

The invention is, however, not restricted to the above water-solublesalts, organic absorbents and concentrations, and further suitablepossibilities will become apparent to those skilled in the art.

As already mentioned above, when combinations of water and theabovementioned organic absorbents are used problems with regard to masstransfer and with regard to leakage are obtained only with, for example,polypropene or polyethene hollow fibres. Surprisingly, it has been foundthat with the aid of the method according to the invention gas/liquidabsorption using, for example, polyethene and polypropene hollow fibremembranes can be operated in a stable manner without any reduction inthe mass transfer coefficient occurring with time, as is obtained withaqueous solutions of monoethanolamine on its own. Moreover it has beenfound, surprisingly, that--compared with the knownmonoethanolamine/water systems--the replacement of part of themonoethanolamine by a water-soluble salt does not lead to a significantreduction in the mass transfer coefficient.

The invention also relates to the use of an aqueous solution of awater-miscible organic absorbent and a watersoluble soluble salt havinga surface tension of at least 60×10⁻³ N/m at 20° C. for membrane gasabsorption, and to a system for membrane gas absorption, comprising ahollow fibre membrane module containing hollow fibres of a materialother than polytetrafluoroethene and a liquid comprising an aqueoussolution of a water-miscible organic absorbent and a water-soluble salt,said solution having a surface tension of more than 60×10⁻³ N/m.

In accordance with the invention, the surface tension of the liquidphase must be brought, by the addition of the water-soluble salt, atleast to a value such that the liquid absorbent does not moisten thepores of the hollow fibre membranes.

For polypropene (Accurel®) the limiting value for the surface tension atroom temperature is 60×10⁻³ N/m. For other membrane materials thistension can be somewhat lower, so that with these materials somewhatlower surface tensions can also be used and this also falls within thescope of the invention.

For practical application, it is, however, preferable that the surfacetension be above the critical surface tension for the membrane material,so that a stable and reliable system is obtained. Moreover, the surfacetension of the liquid phase is dependent on the temperature.

Therefore, in principle, liquid absorbents or mixtures thereof below thelimiting value for the surface tension cannot be used under stableconditions and/or without leakage.

Furthermore, in contrast to known systems, in the case of the inventionsome of the organic absorbent can be replaced by the water-soluble saltwithout this leading to a reduction in the kinetics. As a result, alower concentration of the organic absorbent can be used.

However, in the case of the invention it has been found that the surfacetension of the liquid phase can be increased by the addition of salts,ions or other structure-producing agents without this detracting fromthe kinetics.

The invention therefore makes it possible to make liquid absorbentshaving a low surface tension usable for membrane gas absorption bymixing with other (possibly known) absorbents which increase the surfacetension. This is without out any significant adverse consequences on thereaction kinetics and/or the loading.

In this context, potassium carbonate solutions in general have muchslower kinetics than solutions of monoethanolamine in water. Thestability of the system depends mainly on the mass transfer coefficient.The mass transfer coefficient indicates how much material is transferredper unit time and unit surface area for a constant driving force and istherefore fore an important parameter when sizing the equipment. Thiscoefficient must also remain constant in the case of use for a prolongedperiod.

Another important process parameter is the regeneration energy. Thus,whilst it is true that sodium hydroxide solution and potassium hydroxidesolution give high kinetics, systems of this type demand too high aregeneration energy.

From the abovementioned article by Matsumoto et al., and fromexperiments carried out by the Applicant, it has been found that inexperiments with regard to the CO₂ absorption with 5M MEA (30% inwater), carried out in a hydrophobic polypropene hollow fibre membranemodule, this system shows a high mass transfer coefficient (k>1×10⁻³m/s).

However, it is known from the literature reference that the masstransfer coefficient of this system decreases after some time. Moreover,research by the Applicant has shown that after some time, varying from afew hours to several days, leakage of the liquid phase occurs.

It is not entirely clear what causes this leakage. What is clear is thatthe leakage is not related to too low a breakthrough pressure. The termbreakthrough pressure is used to refer to the pressure under which theliquid phase moistens the pores. The desired breakthrough pressure isabout 1 bar, but is at least 0.5 bar. The breakthrough pressure of a 5MMEA solution is greater than 1.0 bar, the desired breakthrough pressurefor a membrane system. The occurrence of leakage is also not relatedexclusively to the surface tension of the liquid phase, because if thesurface tension of the liquid phase were too low leakage would beexpected immediately on use and not after some time in continuous use.

The method of the invention has been found to be particularly suitablefor the absorption of carbon dioxide from the gas phase. In thiscontext, the water-soluble salt used is preferably a water-solublecarbonate, advantageously sodium carbonate or potassium carbonate usedin a concentration of 1.5-2.5M, preferably 2M. Advantageously, theorganic absorbent used is monoethanolamine in a concentration of0.1-1.5M, preferably 1M.

Finally, the invention can also be used for desorption, in which case agaseous component is transferred from the liquid phase to the gas phase.In general, it can be stated that both in the case of absorption ofgaseous components from the gas phase and in the case of desorption anequilibrium is established between the gas phase and the liquid phase.

The invention also relates to the use of an aqueous solution of awater-miscible organic absorbent and a water-soluble salt, where saidsolution has a surface tension at 20° C. of more than 60×10⁻³ N/m, inmembrane gas absorption, in particular in the absorption of carbondioxide from the gas phase, or the absorption of water vapour from thegas phase.

Finally, the invention relates to a system for membrane gas absorption,comprising a hollow fibre membrane module containing hollow fibres of amaterial other than polytetrafluoroethene and a container, whichcontains an aqueous solution of a water-miscible organic absorbent and awater-soluble salt, where said solution has a surface tension at 20° C.of more than 60×10⁻³ N/m.

The preferences for the above application and the above system are thesame as those for the above method.

B. Amino acids

According to a second preferred aspect of the invention, the liquidphase used is a solution of a water-soluble amino acid or awater-soluble salt thereof.

According to this aspect of the invention, amino acids are understood tobe all organic compounds which contain one or more amino groups and oneor more carboxylic acid groups or sulphonic acid groups.

In this context, the carboxylic acid/sulphonic acid group and the aminogroup can be bonded to the same atom of the organic compound, as in thecase of naturally occurring amino acids, but this is not requiredaccording to the invention. Amino acids in which the amino group and thecarboxylic acid group are separated by two or more atoms, such as carbonatoms, can advantageously be used for the invention.

For use in membrane absorption, these amino acids are subdivided intosterically hindered and non-sterically hindered amino acids, dependingon the accessibility of the amino group for the compound to be absorbed.

These two categories of amino acids follow a different reaction path onabsorption of carbon dioxide. In the case of non-sterically hinderedamino acids, the absorption of carbon dioxide proceeds via the formationof a carbamate via the following reaction equation:

    CO.sub.2 +2RNH.sub.2 →RNH.sub.3 +RNHCOO.sup.31

In the case of sterically hindered amino acids, the absorption of carbondioxide proceeds via the formation of a bicarbonate in accordance with:

    CO.sub.2 +RNH.sub.2 +H.sub.2 O→HCO.sub.3.sup.- +RNH.sub.3 +

In non-sterically hindered amino acids, the amino group and the acidgroup will in general be separated by two or more atoms. Examples ofthese non-sterically hindered amino acids which are preferably used aretaurate and derivatives thereof, which are described in Canadian Patent619 193--the contents of which are incorporated herein by reference--forthe absorption of carbon dioxide in absorption columns. However, saidCanadian patent does not describe the use of taurine and derivativesthereof in membrane gas absorption.

The other non-sterically hindered amino acids which can be used for thepresent invention will be apparent to those skilled in the art; examplesare, inter alia; methyltaurine, methyl-α-aminopropionic acid,N-(β-ethoxy)taurine and N-(β-aminoethyl)taurine, as described inCanadian Patent 619 193, the contents of which are incorporated hereinby reference.

Examples of sterically hindered amino acids are the naturally occurringamino acids--i.e. amino acids which are natural constituents ofproteins--in which acids the accessibility of the amino group to thesubstance to be absorbed is restricted by the presence of an amino groupand a carboxylic acid group on the same carbon atom.

Examples of the above acids are, for example, alanine and glycine, aswell as derivatives thereof, such as dimethylglycine. Aqueous solutionsof such amino acids for use as liquid absorbent are availablecommercially under the trade name Alkazyd N (alanine) and Alkazyd di-K(dimethylglycine).

Furthermore, it is possible to use amino acids which contain severalamino groups per molecule, such as asparagine, glutamine, lysine andhistidine.

The amino acids can optionally be used in the optically active form.Further desired amino acids can be prepared via, for example, theStrecker synthesis, as will be apparent to those skilled in the art.

The sterically hindered amino acids will absorb carbon dioxide in aratio of 1 mol of carbon dioxide per mol of amino group; in the case ofnon-sterically hindered amines, this ratio is 0.5:1 because of thecarbamate reaction path. However, compared with sterically hinderedamino acids, the non-sterically hindered amino acids offer the advantagethat they, in general, display a lower bonding energy for carbon dioxideand therefore are easier to regenerate.

For the invention, the amino acid is used in an amount which iseffective for absorption and the prevention of leakage, in general0.1-10M, preferably 1-6M.

The amino acid solution will in general have an alkaline pH, moreparticularly a pH of 9-13. The use of an alkaline solution of the aminoacid offers the advantage that the majority of the amino groups in theamino acids are available for absorption in the free form, i.e.non-protonated form.

In order to obtain the liquid absorbent, the amino acid is dissolved inwater, the pH being adjusted to a desired value beforehand, during orafter the addition of the amino acid. The amino acid is preferably addedin the form of a water-soluble salt. For non-sterically hindered aminoacids, sodium and potassium salts, more particularly potassium salts,are preferably used. Salts of this type are less desirable withsterically hindered amino acids, which absorb carbon dioxide via thebicarbonate mechanism, because a bicarbonate precipitate can form onabsorption of carbon dioxide.

The amino acids can optionally be combined with other water-solublesalts, such as carbonate salts. In this case, the relativeconcentrations of the salts can be so chosen that an optimum combinationof transfer coefficient and absorption capacity is obtained, the aminoacid constituent making a greater contribution to the transfercoefficient, whilst the carbonate constituent makes a greatercontribution to the absorption capacity. However, the amino acidsolutions do not have to contain organic absorbents in addition to theamino acid, because the amino acid serves as the absorbent.

The surface tension of the amino acid solutions used will in general andpreferably be above the limiting value of the membrane used, i.e.60×10⁻³ N/m at 20° C. for Accurel fibres. If desired, the surfacetension of the amino acid solution can be further increased by theaddition of a water-soluble salt, as mentioned above.

Further advantages of the use of amino acid solutions compared with, forexample, monoethanolamine/water--in addition to preventing and/orcounteracting leakage--are:

amino acids are less corrosive;

amino acids are more stable to O₂ :

the absorption capacity is comparable to that of, for example,monoethanolamine;

the regeneration energy is comparable to that of monoethanolamine, and

there is no reduction in the mass transfer coefficient with time.

The amino acid solutions in question can therefore be used forapplications where the use of organic solvents or corrosive salts andcombinations thereof is undesirable, such as applications at elevatedtemperatures higher than 70° C. For these applications the amino acidliquid phases provide a valuable alternative to the liquid phasescontaining organic absorbent.

The invention also relates to the use of an aqueous solution of awater-soluble amino acid or a water-soluble salt thereof in membrane gasabsorption for the prevention of leakage.

Finally, the invention relates to a system for membrane gas absorption,comprising a hollow fibre membrane module containing hollow fibres of amaterial other than polytetrafluoroethene, and a container, whichcontains an aqueous solution of a water-soluble amino acid or awater-soluble salt thereof.

c. Phosphate salts

According to a third preferred aspect of the invention, the liquid phaseused is an aqueous solution of a water-soluble phosphate salt.

Any water-soluble phosphate salt can be used for this purpose, sodiumphosphate, potassium phosphate and ammonium phosphate, and in particularpotassium phosphate, being preferred.

The phosphate salts will in general be used in a concentration which iseffective in respect of the prevention or counteraction of leakage andwhich gives the desired absorption capacity, mass transfer and kinetics.This concentration will in general be 0.5-5M, preferably about 2M.

The phosphate salts will in general be used in alkaline solution, moreparticularly at a pH of 9-13. This means that the majority of thephosphate anions in this solution will be present in the PO₄ ³⁻ form.

In order to obtain the liquid phase, the phosphate salt is dissolved inwater, after which the pH is, if necessary, adjusted to a desired value.In this context it is preferable to add a phosphate salt which does notcontain any H⁺ ions, although it is also possible to add monohydrogenphosphate and dihydrogen phosphate and then to bring the pH to thedesired alkaline value, the trivalent phosphate ions being obtained.

The phosphate salt used according to the invention serves as absorbent,so that it is not necessary to add any organic absorbents in addition tothe phosphate salts.

The surface tension of the phosphate solutions will in general andpreferably be higher than the limiting value for the membrane used, i.e.higher than 60×10⁻³ N/m at 20° C. for, for example, Accurel fibres.

The phosphate salts used according to the invention are in particularsuitable for the removal of carbon dioxide discharge streams. Anothervery suitable application of the phosphate salts is the removal ofhydrogen sulfide from a gaseous stream, as described in U.S. Pat. No.1,945,163--the contents of which are incorporated here by reference--incolumn gas absorption. However, the said patent does not describe theuse of phosphate salts in membrane gas absorption.

Further advantages of the use of phosphate salts are:

phosphate salts are less corrosive;

phosphate salts are more stable to O₂ ;

the absorption capacity is comparable to that of, for example,monoethanolamine;

the regeneration energy is comparable to that of monoethanolamine, and

there is no reduction in the mass transfer coefficient with time.

The invention also relates to the use of an aqueous solution of awater-soluble phosphate salt in membrane gas absorption for theprevention of leakage.

Finally, the invention relates to a system for membrane gas absorption,comprising a hollow fibre membrane module containing hollow fibres of amaterial other than polytetrafluoroethene, and a container, whichcontains an aqueous solution of a water-soluble phosphate salt.

The invention and the preferred aspects mentioned above will beexplained below on the basis of the following examples, which, however,do not restrict the scope of the invention.

EXAMPLE 1

This example describes the removal of carbon dioxide from a mixture withnitrogen (6% CO₂) at room temperature using an aqueous solution of 1Mmonoethanolamine and 2M potassium carbonate, making use of porous hollowfibre membranes (Accurel®, polypropene, external diameter 1 mm, internaldiameter 0.6 mm in Microdyn module LM2PO6).

During the (long-term) experiment, the liquid absorbent was pumpedcontinuously through the lumen of the fibre at a low flow rate (0.2l/min) and a low excess pressure on the liquid side (0.04 bar). Noleakage was detected during the experiment.

Furthermore, the mass transfer coefficient was determined at threepoints in time during the 8-day test period. This determination wascarried out by feeding the gas mixture described through the membranemodule (outside the fibres) and measuring CO₂ concentrations at theinlet and outlet. The following table shows that the mass transfercoefficient remained constant during the test period, which signifiesthat no leakage took place.

                  TABLE 1                                                         ______________________________________                                        Mass transfer coefficient with porous polypropene hollow                      fibre membranes (microdyn/Accurel fibres); gas: 6% CO.sub.2 in                N.sub.2 ; liquid absorbent: 1M monoethanolamine, 2M K.sub.2 CO.sub.3 ;        gas                                                                           flow rate: 5 l/min.; liquid flow rate: 0.14 l/min.; liquid                    excess pressure: 0.04 bar.                                                               Mass transfer coefficient                                          Day         10.sup.-3 m/s!                                                    ______________________________________                                        1          1.06                                                               5          1.01                                                               8          1.02                                                               ______________________________________                                    

EXAMPLE 2

This example describes the removal of carbon dioxide from a mixture withnitrogen (6% CO₂) at room temperature using an aqueous solution of 2Mtaurine and potassium hydroxide solution added to give pH 11.68, makinguse of porous hollow fibre membranes (Accurel®, polypropene, externaldiameter 1 mm, internal diameter 0.6 mm in Microdyn module LM2PO6).

During the (long-term) experiment, the liquid absorbent was pumpedcontinuously through the lumen of the fibre at a low flow rate (0.14l/min) and a low excess pressure on the liquid side (0.04 bar). Noleakage was detected during the experiment.

In addition, the mass transfer coefficient was determined at four pointsin time during the 9-day test period. This determination was carried outby feeding the gas mixture described through the membrane module(outside the fibres) and measuring CO₂ concentrations at the inlet andoutlet. The following Table 2 shows that the mass transfer coefficientremained constant during the test period, which signifies that noleakage took place.

                  TABLE 2                                                         ______________________________________                                        Mass transfer coefficient with porous polypropene hollow                      fibre membranes (Microdyn/Accurel fibres); gas: 6% CO.sub.2 in                N.sub.2 ; liquid absorbent: 2M taurine and potassium hydroxide                solution added to give pH 11.68; gas flow rate 5 l/min;                       liquid flow 0.14 l/min; liquid excess pressure 0.04 bar.                                 Mass transfer coefficient                                          Day         10.sup.-3 m/s!                                                    ______________________________________                                        1          1.09                                                               3          1.00                                                               5          1.02                                                               9          0.96                                                               ______________________________________                                    

We claim:
 1. Method for the absorption of one or more gaseous componentsfrom a gas phase in a membrane system, in that the gas phase with thecomponent(s) to be absorbed present therein is brought into contact witha liquid phase, wherein the gas phase and the liquid phase are separatedby a hydrophobic membrane, and the gaseous components are absorbed intosaid liquid phase through the hydrophobic membrane and then removed fromthe membrane system while absorbed in said liquid phase, whereinthegaseous components are chosen from carbon dioxide and/or hydrogensulfide; the membranes are composed of polypropene, polyethene,polyvinylidene fluoride or polysulfone; the liquid phase comprises waterand a water-miscible and/or water-soluble absorbent; the liquid phasehas a surface tension at 20° C. of more than 60×10⁻³ N/m; the liquidphase does not give rise to any leakage from the membrane or iseffective in preventing or counteracting leakage from the membrane;withthe proviso that the liquid phase is not an aqueous solution consistingsolely of monoethanolamine (MEA) and water.
 2. Method according to claim1 characterised in that the liquid phase comprises an aqueous solutionof a water-soluble amino acid or a water-soluble salt thereof.
 3. Methodaccording to claim 2, characterised in that the amino acid contains acarboxylic acid group or a sulphonic acid group.
 4. Method according toclaim 2 characterised in that the solution of the amino acid or the saltthereof has a pH of 9-13.
 5. Method according to one of claim 2,characterised in that the amino acid is present in a concentration of0.1-10M.
 6. Method according to one of claim 2, characterised in thatthe amino acid is a non-sterically hindered amino acid.
 7. Methodaccording to claim 6, characterised in that the amino acid is taurine ora derivative thereof.
 8. Method according to one of claim 2,characterised in that the amino acid is selected from the naturallyoccurring amino acids.
 9. Method according to one of claim 1,characterised in that the liquid phase comprises an aqueous solution ofa water-soluble phosphate salt.
 10. Method according to claim 9,characterised in that the solution of the water-soluble phosphate salthas a pH of 9-13.
 11. Method according to claim 9, characterised in thatthe phosphate salt is present in a concentration of 0.5-5M.
 12. A methodfor the absorption of one or more gaseous components from a gas phase ina membrane system, wherein the gas phase containing the component(s) tobe absorbed is brought into contact with a liquid phase, wherein the gasphase and the liquid phase are separated by a hydrophobic membrane, andthe gaseous components are absorbed into said liquid phase through thehydrophobic membrane and then removed from the membrane system whileabsorbed in said liquid phase, whereinthe gaseous components areselected from the group consisting of carbon dioxide, hydrogen sulfide,and mixtures thereof; the membranes are composed of polypropene,polyethene, polyvinylidene fluoride or polysulfone; the liquid phasecomprises water and an absorbent which is selected from the groupconsisting of water-miscible absorbent, water-soluble absorbents, andmixtures thereof; the liquid phase has a surface tension at 20° C. ofmore than 60×10⁻³ N/m; the liquid phase does not give rise to anyleakage from the membrane or is effective in preventing or counteractingleakage from the membrane; with the proviso that the liquid phase is notan aqueous solution consisting solely of monoethanolamine and water andwith the further proviso that the membrane is not a liquid membrane. 13.A method for absorption, in a membrane system, of at least one gaseouscomponent from a gas phase into a liquid phase, said membrane systemhaving at least one hydrophobic membrane permeable to said at least onegaseous component, which membrane system defining at least one gas feedchannel on one side of the membrane and at least one liquid feed channelon the opposite side of the membrane, wherein the gas phase with thecomponents to be absorbed present therein is led into the gas feedchannel and brought into contact with one side of the membrane, and theliquid phase is led into the liquid feed channel and brought intocontact with the opposite side of said membrane, in such a way that theat least one gaseous component is absorbed from the gas phase into theliquid phase through the hydrophobic membrane; the gas phase and theliquid phase being separated by the hydrophobic membrane, and the liquidphase containing the one or more gaseous components absorbed therein isthen removed from the membrane system, whereinthe gaseous components areselected from the group consisting of carbon dioxide, hydrogen sulfide,and mixtures thereof; the membranes are made of polypropene, polyethene,polyvinylidene fluoride or polysulfone; the liquid phase comprises waterand an absorbent selected from the group consisting of water solubleabsorbents, water miscible absorbents, and mixtures thereof; the liquidphase has a surface tension at 20° C. of more than 60×10⁻³ N/m; theliquid phase does not leak from the membrane; with the proviso that theliquid phase is not an aqueous solution consisting solely ofmonoethanolamine and water.
 14. A method for absorption, in a membranesystem, of at least one gaseous component from a gas phase into a liquidphase, said membrane system having at least one hydrophobic membranepermeable to said at least one gaseous component, which membrane definesat least one gas feed channel on one side of the membrane and at leastone liquid feed channel on the opposite side of the membrane, whereinthe gas phase with the at least one component to be absorbed presenttherein is led into the gas feed channel and brought into contact withone side of the membrane, and the liquid phase is led into the liquidfeed channel and brought into contact with the opposite side of saidmembrane, in such a way that the at least one gaseous component isabsorbed from the gas phase into the liquid phase through thehydrophobic membrane;the gas phase and the liquid phase being keptseparated by the hydrophobic membrane, and the liquid phase containingthe at least one gaseous component absorbed therein is removed from themembrane system, wherein the gaseous components are selected from thegroup consisting of carbon dioxide, hydrogen sulfide, and mixturesthereof; the membranes are made of polypropene, polyethene,polyvinylidene fluoride or polysulfone; the liquid phase comprises waterand an absorbent selected from the group consisting of water solubleabsorbents, water miscible absorbents, and mixtures thereof; the liquidphase has a surface tension at 20° C. of more than 60×10⁻³ N/m; theliquid phase does not leak from the membrane; with the proviso that theliquid phase is not an aqueous solution consisting solely ofmonoethanolamine and water; and with the further proviso that themembrane is not a liquid membrane.
 15. A method for the absorption, in amembrane system, of one or more gaseous components from a gas phase,said system havinga wall surrounding a space, said space being providedwith at least one hydrophobic membrane, which is permeable to said atleast one gaseous component; said at least one membrane defining, withinsaid space, at least one liquid channel and at least one gas channel; atleast one liquid inlet and and least one liquid outlet operablyconnected to said at least one liquid channel; at least one gas inputand at least one gas output operably connected to said at least one gaschannel; wherein a gas phase with the at least one component to beabsorbed present therein is introduced via the at least one gas inputinto the at least one gas channel; wherein a liquid phase is introducedinto the at least one liquid channel via the at least one liquid input;wherein in such a way that the gas phase is brought into contact withthe liquid phase, and the at least one gaseous component is absorbedfrom the gas phase into the liquid phase through the hydrophobicmembrane, the gas phase and the liquid phase being separated by thehydrophobic membrane; wherein the liquid phase with the at least onegaseous component absorbed therein is removed from the membrane systemvia the at lease one liquid output, and the gas phase, from which the atleast one gaseous component is at least partially removed, is removedfrom the reactor via the at least one gas outlet, wherein the gaseouscomponents are selected from the group consisting of carbon dioxide,hydrogen sulfide, and mixtures thereof; the membranes are made ofpolypropene, polyethene, polyvinylidene fluoride or polysulfone; theliquid phase comprises water and an organic absorbent selected from thegroup consisting of water soluble absorbents, water miscible absorbents,and mixtures thereof; the liquid phase has a surface tension at 20° C.of more than 60×10⁻³ N/m; the liquid phase does not leak from themembrane; with the proviso that the liquid phase is not an aqueoussolution consisting solely of monoethanolamine and water.
 16. Methodaccording to claim 15, wherein the liquid phase contains a water-solublesalt which is selected from water-soluble carbonates.
 17. A methodaccording to claim 16 wherein the water-soluble carbonate is selectedfrom the group consisting of potassium carbonate and sodium carbonate.18. Method according to claim 15, characterised in that the organicabsorbent is selected from monoethanolamine, diethanolamine (DEA),methyldiethanolamine (MDEA), methyl ethyl ketone (MEK), methyl isobutylketone (MBK), (poly)ethene glycols, ethers, alcohols andN-methylpyrrolidone.
 19. Method according to one of claims 15,characterised in that the organic absorbent is used in a concentrationof 0.05-10M.
 20. Method according to claim 15, wherein the liquid phasecontains a salt, which is used in a concentration of 0.05-10M. 21.Method according to claim 15, wherein the liquid phase contains a saltand said salt is a water-soluble carbonate and the organic absorbentused is a monoethanolamine.
 22. A method according to claim 21 whereinthe water-soluble carbonate is selected from the group consisting ofpotassium carbonate and sodium carbonate.
 23. Method according to claim15, wherein a water-soluble carbonate is present in the liquid phase ina concentration of 1.5-2.5M and the monoethanolamine is used in aconcentration of 0.5-1.5M.